The present invention relates to a filter circuit, and particularly to an all-pass filter circuit for use in a delay circuit.
As is known well, filters are classified into a low pass filter (LPF), a high pass filter (HPF) and so forth depending upon which frequency is selected. The low pass filter allows a direct current (DC) signal to a signal having a predetermined frequency to pass through while preventing a signal having more than the predetermined frequency. On the other hand, the high pass filter allows signals having frequencies higher than the predetermined frequency to pass through while preventing signals having frequencies lower than the predetermined frequency.
FIG. 1A shows a low pass filter constituted of the most simple CR (capacitor and resistance). A voltage of FIG. 1A is given by an equation (1) EQU I=(V1-V2)/R=j.omega.CV.sub.2 (1)
The voltage ratio between input and output is given from the equation (1) as follows: EQU V2/V1=1/(1+j.omega.CR) (2)
The equation (2) is called a transfer function of the low pass filter.
FIG. 1B is a high pass filter constituted of the most simple CR circuit. The transfer function is similarly given below: EQU V2/V1=j.omega.CR/(1+j.omega.CR) (3)
On the other hand, an all-pass filter (APF) is known. The APF changes a phase or transfer time of a signal without affecting amplitude. This is used for correcting image disturbance due to distortion in phase or delay time in a wave transfer circuit of a TV tuner, VTR and the like. The transfer function is given by an equation (4): EQU V2/V1=(1-j.omega.CR)/(1+j.omega.CR) (4)
In a conventionally-used integrated circuit, all pass filer (APF) is realized by forming an operation amplifier using a bipolar transistor, as shown in FIG. 2. More specifically, signals, (Vin+Vo)/2 and (Vin/(1+j.omega.C.sub.1 R.sub.1) are input to two input ports of an operation amplifier and then output (1-j.omega.C.sub.1 R.sub.1)/(1+j.omega.C.sub.1 R.sub.1) is obtained.
When an analog filter is realized in the recent digitalization trend in electronic appliances, it is a general way to form an integrated circuit having bipolar transistors for realizing the analog filter and MOS transistors for attaining digital processing, in combination. However, a manufacturing process of such an integrated circuit is complicated, thus increasing cost.